Type ContentPage.Padding not found in xmlns http://xamarin.com/schemas/2014/forms

I'm getting this error when trying to run one of the XAML demos, trying to add padding to a content page for iOS:

<ContentPage.Padding>
    <OnPlatform x:TypeArguments="Thickness"
                iOS="0, 20, 0, 0" />
  </ContentPage.Padding>